Analytical chemistry, like many other scientific and industrial fields, is experiencing the challenge to be more sustainable by using renewable resources. For GC users, it means finding alternatives to the use of helium as a carrier gas, especially in light of the recent helium shortage and price increase.
The choice of carrier gas in gas chromatography affects the efficiency, speed and cost of the analysis. The most commonly used carrier gases are helium, nitrogen and hydrogen. Each has advantages and disadvantages, but hydrogen has emerged as an attractive alternative to helium, due to its benefits for GC and GC–MS (mass spectrometry) applications: Hydrogen has a higher optimal linear velocity than helium, which means shorter analysis times and higher productivity without compromising efficiency.
Additionally, it maintains high efficiency over a wider range of linear velocities, giving more flexibility in adjusting the flow rate and temperature conditions to optimize the separation.
Hydrogen is also an attractive choice because it is less expensive than helium, is renewable and readily available since it can be generated on-site from water using a hydrogen generator. This reduces the dependence on external suppliers and eliminates the need for storing high-pressure cylinders.
However, migrating to carrier hydrogen for GC and GC-MS applications also has some drawbacks, or at least it requires precautions and method optimization. The main concern is safety, as hydrogen is flammable and explosive. Therefore, proper actions must be taken to limit discharge in the ambient, to avoid leaks, sparks and ignition sources. Also, hydrogen is not inert and may react with some analytes — such as halogenated compounds — and alter their chromatographic behavior and response. Moreover, when hydrogen is used under vacuum with a GC-MS, a suitable pumping system and narrow bore columns are required to properly handle its low viscosity. A non-suitable vacuum in the ion source and high volumetric hydrogen flow through the column not only impacts on sensitivity, but may pose a risk of hydrogen accumulation in the MS system.
How HeSaver-H2Safer option can help
To make hydrogen carrier gas usage safer and more efficient for GC and GC-MS applications, the Thermo Scientific™ HeSaver-H2Safer™ technology is very helpful. It works on an adapted split/splitless injector (SSL) by using an inert gas such as nitrogen to pressurize the injector and maintain the split and purge flows. Hydrogen is supplied to the analytical column with only a limited maximum flow rate. The limited gas consumption of hydrogen is beneficial for operational safety. The amount of hydrogen present in the system is so small that there is no risk to reach hazardous concentrations even in case of column breakage, and therefore a hydrogen sensor in the GC oven is not necessary.
Additionally, no hydrogen is discharged in the ambient through the split and purge lines, thanks to the inert pressurizing gas. The pressurizing gas also offers the benefit of eliminating the contact between the analytes and hydrogen carrier in the hot injector and consequently decreases the possibility of unwanted reactions.
Another benefit of using nitrogen as the pressurizing gas is to improve sample transfer into the column. Especially when working with a GC-MS, the low viscosity of hydrogen, combined with the vacuum column outlet, will generate a much lower head pressure with a consequently slower sample transfer into the column and risk of liner backflush during solvent evaporation, especially with splitless injection. Injecting and transferring the sample with nitrogen mitigates those effects thanks to its higher viscosity.
HeSaver-H2Safer SSL injector working with carrier hydrogen and nitrogen as pressurizing gas.
Pesticides analysis with carrier hydrogen
A very common and critical GC-MS application is the analysis of pesticide residues in food. The complexity and variability of food matrices and the regulatory maximum required limit (MRL) impose the use of very selective and sensitive instrumentation. The Thermo Scientific™ TSQ™ 9610 Triple Quadrupole GC-MS/MS equipped with the Never Vent Advance EI ionization source meets and exceeds the required analytical performance, combining compliance to official regulations with extended robustness and instrument uptime.
Helium carrier gas ensures optimum analytical performance. When combined with the HeSaver-H2Safer technology, it offers the opportunity to significantly reduce helium gas consumption, significantly extending helium cylinder lifetime and mitigating helium shortage issues.
However, for a growing number of laboratories hydrogen is the only option, with the need to re-optimize conditions and re-validate their methods. To support laboratories in this conversion, the operative conditions of TSQ 9610 AEI GC-MS/MS have been optimized for the analysis of pesticides using hydrogen as carrier gas. More than 180 pesticides have been tested and the transitions optimized with Thermo Scientific AutoSRM software to obtain the highest possible sensitivity. Additionally, the use of hydrogen improved the chromatography and allowed to set narrower retention time windows, which contributed to a shorter dwell time, thus helping to increase sensitivity and compensate losses typically occurring with the use of hydrogen as carrier gas.
For some of the analytes, the presence of hydrogen affected the ionization process. In other words, the spectrum obtained with hydrogen was different than the spectrum acquired with helium. However, this difference should not be considered a critical issue, as targeted methods used for pesticide residue analysis rely on compound identification based on retention time and ion ratios between compound-specific transitions, and not on the fidelity to spectral libraries. The sensitivity of the final method was good enough to quantify the pesticides at 0.005 mg/kg, in full compliance to SANTE criteria. In conclusion, hydrogen is a viable carrier gas for GC and GC–MS offering several advantages over helium in terms of speed, efficiency, cost and sustainability. Its use in combination with the HeSaver-H2Safer technology eliminates some of the drawbacks. However, it still implies careful attention to compatibility issues. Therefore, before switching to hydrogen as a carrier gas, you are advised to perform validation tests to ensure optimal performance and quality.